Barrier layer fabrication methods

ABSTRACT

A process for forming a storage capacitor for a semiconductor assembly, by forming a first storage electrode having a top surface consisting of titanium nitride; forming a barrier layer directly on the titanium nitride, the barrier layer (a material containing any one of amorphous silicon, tantalum, titanium, or strontium) being of sufficient thickness to substantially limit the oxidation of the titanium nitride when the semiconductor assembly is subjected to an oxidizing agent (either an oxidizing agent or an nitridizing agent); converting a portion of the barrier layer to a dielectric compound; depositing a storage cell dielectric directly on the dielectric compound, the storage cell dielectric being of the same chemical makeup as the dielectric compound and thereby using the dielectric compound as a nucleation surface; and forming a second capacitor electrode on the storage cell dielectric.

This application is a continuation to U.S. patent application Ser. No.09/004,932, filed Jan. 9, 1998 now U.S. Pat. No. 6,180,481.

FIELD OF THE INVENTION

This invention relates to semiconductor fabrication processing and moreparticularly to a method for fabricating oxygen diffusion barrier layersto enhance the electrical characteristics of a storage capacitor for aDynamic Random Access Memory (DRAM).

BACKGROUND OF THE INVENTION

The continuing trend of scaling down integrated circuits has forced thesemiconductor industry to consider new techniques for fabricatingprecise components at sub-micron levels. With the industry movingtowards processes for fabrication of smaller device geometries,isolation between devices becomes a very critical issue.

Fabrication processes for fabrication of DRAMs having smaller devicegeometries are of particular interest as each new generation of DRAMspush technology, including equipment limitations, to new areas ofresearch. A major area of research has been in the development of theDRAM storage cell capacitor. Each DRAM generation requires a device thatis more dense than the preceding generation and yet industry andcompetition demand that the physical size of the device remaincomparable in size to the preceding generation. To accomplish thisrequirement, much of the DRAM research is focused on building a smallerstorage capacitor and yet maintain adequate capacitance, as the storagecapacitor requires a good portion of the DRAM dice.

One method of increasing capacitance without increasing the storageelectrode size is by providing a storage cell dielectric that possesseshigh quality dielectric characteristics, such as a high dielectricconstant, etc. The quality of a cell dielectric is also somewhatdependent on the type of material used to build the capacitor electrodesdue mainly in part to the work function potential of the material. (Thework function potential of a material is the potential of a barrierwhich must be overcome to remove an electron from the fermi-level to thevacuum level outside the material.) Electrodes made from titaniumnitride (TiN) are desirable for use in DRAM capacitor due to the largebarrier height (which will deter current conduction resulting from thework function potential of the TiN) that develops at the TiN/celldielectric interface. This will potentially lead to leakage reduction ina dielectric dominated by electrode-limited conduction properties. Thedielectric leakage current mechanism is dominated by the materialproperties of the capacitor electrodes rather than the dielectricitself, as the leakage current attributed to the characteristics of adielectric is minimal when compared to the leakage current attributed tothe material make-up of the capacitor electrodes themselves.

However, TiN electrodes are prone to an oxidation of the top 70 to 430 Åwhen exposed to an oxygen atmosphere at elevated temperatures. Thisoxidation of the TiN surface leads to poor nucleation of the celldielectric which results in an increase in possible leakage currentthrough the dielectric and a reduction in the reliability of thecapacitor. This reduction in electrical quality has been shown to existin both silicon nitride and tantalum oxide (Ta₂O₅) dielectrics depositedon TiN electrodes. For example, Si₃N₄ forms an excellent interface whenit is deposited directly on bare silicon, but forms a poor interfacewhen deposited on refractory metal nitrides, such as TiN, or onrefractory metals themselves.

Typically, for a capacitor utilizing a Ta₂O₅ storage cell dielectric,the Ta₂O₅ is deposited on a lower capacitor electrode made up of asilicon nitride layer, formed by rapid thermal processing (RTP), whichin turn is formed on top of polysilicon. This configuration creates apolysilicon/nitride/Ta₂O₅ interface. If the top capacitor electrode isTiN, a top Ta₂O₅/TiN interface is formed. This particular capacitorstructure, consisting of a polysilicon/nitride/Ta₂O₅ interface, createsa lower barrier to leakage current then does the barrier created by theTa₂O₅/TiN interface. Therefore the polysilicon/nitride/Ta₂O₅ interfaceallows a larger leakage current from the capacitor during a positivebias then does the Ta₂O₅/TiN interface. Due to this lower barrier heightat the polysilicon/nitride/Ta₂O₅ interface the polysilicon/nitride/Ta₂O₅interface now becomes the limiting factor in determining the performanceof the capacitor.

Implementations of the present invention teach methods to efficientlyuse conductive materials, such as silicon, noble metals, refractorymetals, refractory metal nitrides and in particular TiN, as the lowercapacitor electrode and to avoid the above limitation (resulting fromconventional processes). In the case of using TiN as the capacitorbottom electrode, conventional processes deposit a cell dielectricdirectly on the TiN electrode. However, these methods do not effectivelylimit the oxidation of the TiN prior to cell dielectric deposition. Anembodiment of the present invention provides a reliable method toeffectively use the conductive materials listed above as capacitorbottom electrode while providing a dielectric nucleation surface for asubsequent deposition of cell dielectric.

SUMMARY OF THE INVENTION

Exemplary implementations of the present invention disclose methods forchemically bonding a dielectric material to a conductive material in asemiconductor fabrication process. The process converts at least aportion of a conductive layer that has been formed directly on theconductive material to a dielectric compound, thereby forming adielectric nucleation surface. After the nucleation surface is formed adielectric film containing chemical elements that make the conductivelayer is deposited directly on the dielectric nucleation surface. Theconverted portion of the conductive layer becomes a barrier layer tooxygen diffusion when the structure is subjected to a subsequent oxygenambient. The process flirter provides suitable methods to fabricatecapacitors that utilize conductive materials, such as, silicon,refractory metals, refractory metal nitrides, and noble metals, as thebottom electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view depicting a capacitor storageelectrode on a semiconductor substrate where the electrode consists of atitanium nitride surface on top of which is a layer of tantalum.

FIG. 1B is a cross-sectional view continuing with process steps shown inFIG. 1A, depicting the tantalum layer being converted to tantalum oxideduring an oxygen anneal.

FIG. 1C is a cross-sectional view continuing with process steps shown inFIG. 1B, depicting the deposition of a tantalum oxide cell dielectricfollowed by the formation of a top titanium nitride electrode.

FIG. 2A is a cross-sectional view depicting a capacitor storageelectrode on a semiconductor substrate where the electrode consists of atitanium nitride surface on top of which is a layer of amorphoussilicon.

FIG. 2B is a cross-sectional view continuing with process steps shown inFIG. 2A, depicting the amorphous silicon layer being converted tosilicon nitride during an nitrogen anneal.

FIG. 2C is a cross-sectional view continuing with process steps shown inFIG. 2B, depicting the deposition of a silicon nitride cell dielectricfollowed by the formation of a top titanium nitride electrode.

FIG. 3A is a cross-sectional view depicting a capacitor storageelectrode on a semiconductor substrate where the electrode consists of atitanium nitride surface on top of which is a layer of astrontium/titanium compound.

FIG. 3B is a cross-sectional view continuing with process steps shown inFIG. 3A, depicting the strontium/titanium layer being converted to anoxide (STO) during an oxygen anneal.

FIG. 3C is a cross-sectional view continuing with process steps shown inFIG. 3B, depicting the deposition of a high dielectric constant celldielectric followed by the formation of a top titanium nitrideelectrode.

DETAILED DESCRIPTION OF THE INVENTION

Several exemplary implementations of the present invention are directedto a process for forming a capacitor cell dielectric as depicted in theembodiments of FIGS. 1A-1C, FIGS. 2A-2C, and FIGS. 3A-3C.

In a first exemplary implementation of the present invention andreferring now to FIG. 1A, a cross-sectional view depicts a capacitorstorage electrode 11 formed on semiconductor substrate 10. In this casesemiconductor substrate 10 is making reference to a material whichsupports the construction of the capacitor electrode 11. Capacitorelectrode 11 consists of a polysilicon portion 12 on top of which isformed a titanium nitride (TiN) layer so that capacitor electrode 11 hasa titanium nitride surface 13. Although polysilicon is shown and is apreferred material used to construct the lower portion of capacitorelectroce 11 other materials may be used that are suitable for acceptinga layer of TiN as the surface layer. After the TiN is formed, aconductive tantalum barrier layer 14 is formed directly on the TiN layer13. It is desired that the thickness of tantalum layer 14 be optimizedso that it minimizes or even eliminates oxidation of underlying TiNlayer 13 during subsequent exposure to oxygen, such as an oxidationanneal step or the dielectric deposition step.

Referring now to FIG. 1B, a portion of tantalum barrier layer 14 isconverted to tantalum oxide 15 during an oxygen anneal. The conversionof tantalum to tantalum oxide may either be performed by an oxygenanneal while the assembly is in a deposition chamber during a subsequenttantalum oxide deposition step or by a separate rapid thermal oxidationstep prior to tantalum oxide deposition. It is desired that the TiNsurface be oxidized no more than a maximum of 50 Å and it is furtherpreferred that essentially none (0%) of the TiN surface be oxidized.Regardless of the method used to oxidize the tantalum, major advantagesof the present invention are gained. One advantage is, during formationthe tantalum will make a strong chemical bond to the underlying TiN.Another advantage is, during oxidation only a portion of the tantalumbecomes oxidized while protecting the TiN from becoming oxidized. Theoxidation of the tantalum maintains a strong chemical bond to the TiNand also just as advantageous, provides a tantalum oxide nucleationsurface for a subsequent tantalum oxide deposition step.

Referring now to FIG. 1C, tantalum oxide cell dielectric 16 is depositedon tantalum oxide nucleation surface 15. Nucleation surface 15 may becreated by various methods. Am efficient method is to use the chemicalreactions that occur during deposition of the tantalum oxide itself.During deposition, the chamber contains oxygen atoms that will initiallyoxidize the tantalum surface and thus create nucleation surface 15. Asdeposition of the tantalum oxide continues less and less of tantalumbarrier layer will become oxidized. Another method would be todeliberately oxidize tantalum barrier layer 14 prior to tantalum oxidedeposition and thereby create nucleation surface 15. In either case, itis important that only a portion of tantalum barrier layer 14 beconverted to tantalum oxide so that the remaining portion of thenon-converted (non-oxidized) tantalum barrier layer 14 is sufficient inthickness so that a subsequent oxygen anneal will only oxidize theremaining non-converted portion of barrier layer 14 and thus prevent orsubstantially restrict the oxidation of underlying TiN layer 13.Regardless of which method is used to create nucleation surface 15, thepresence of tantalum barrier layer 14 causes the formation of anucleation surface that is chemically the same as the deposited celldielectric (in other words in this example, both the barrier layer andthe cell dielectric contain tantalum elements).

The deposited tantalum oxide 16 forms a strong chemical bond to thetantalum oxide nucleation surface 15 and thereby creates a high qualitycell dielectric film which has been shown to reduce charge leakage toacceptable levels for DRAM operation depending on the tantalum oxidefilm thickness (less than 10-7 A/cm² at 1 volt for maximum capacitance).Finally the capacitor is completed by the formation of a top electrode17. It is preferred that electrode 17 be titanium nitride or tungstennitride, however other materials used by those skilled in the art toform the top capacitor electrode may be used.

In a second exemplary implementation of the present invention andreferring now to FIG. 2A, a cross-sectional view depicts a capacitorstorage electrode 21 on semiconductor substrate 20. In this casesemiconductor substrate 20 is making reference to a material whichsupports the construction of the capacitor electrode 21. Capacitorelectrode 21 consists of a polysilicon portion 22 on top of which isformed TiN so that capacitor electrode 21 has a TiN surface 23. Althoughpolysilicon is shown and is a preferred material used to construct thelower portion of capacitor electrode 21, other materials may be usedthat are suitable for accepting a layer of TiN as the surface layer.After the TiN is formed, a conductive layer 24 of amorphous silicon isformed directly on TiN layer 23 at a preferred thickness no greater than50 Å. The thickness of the amorphous silicon layer is critical as itmust be thin enough to be entirely consumed during a subsequent nitrogenanneal to form silicon nitride and yet thick enough to protect theunderlying TiN layer 23 from becoming oxidized during a following annealstep. The thin silicon layer can be deposited at temperatures below 525°C. where the temperature is at a low enough point where TiN is unlikelyto oxidize.

Referring now to FIG. 2B, amorphous silicon layer 24 is converted tosilicon nitride 25 during an nitrogen anneal. The conversion ofamorphous silicon to silicon nitride may be performed by a separaterapid thermal nitridation (RTN) step or the amorphous silicon layer 24may be converted to silicon nitride during a silicon nitride depositionstep. If conversion of the layer 24 is relied on during silicon nitridedeposition, nitrogen atoms from a source gas supplied to the depositionchamber, such as NH₃, will initially start to combine with the amorphoussilicon to form silicon nitride. If this method alone is used tonitridize the amorphous silicon, the deposition step must continue longenough to completely convert the amorphous silicon to silicon nitrideand thus avoid formation of a resistive interface between the TiN andamorphous silicon. It is preferred however, that the amorphous siliconis subjected to an RTN step to insure complete nitridation of theamorphous silicon.

Among the advantages of the present invention, the amorphous silicon,will initially make a strong chemical bond to the underlying TiN duringthe formation of the amorphous silicon. As another advantage, during thenitridation of the amorphous silicon, the amorphous silicon is convertedto silicon nitride which will protect the TiN from becoming oxidizedupon exposure to oxygen, such as to air or during the reoxidationprocess which typically follows nitride deposition. The nitridation ofthe amorphous silicon maintains a strong chemical bond to the TiN andalso advantageously, provides an silicon nitride nucleation surface fora subsequent silicon nitride deposition.

Referring now to FIG. 2C, silicon nitride cell dielectric 26 isdeposited on silicon nitride nucleation surface 25. As mentioned above,a nucleation surface 25 is created that is chemically the same as thedeposited cell dielectric. Thus, the deposited silicon nitride forms astrong chemical bond to the silicon nitride nucleation surface andthereby creates a high quality cell dielectric film which has been shownto reduce charge leakage to result in capacitors with excellentreliability and electrical performances. Finally the capacitor iscompleted by the formation of a top electrode 27. It is preferred thatelectrode 27 be TiN, however other materials used by those skilled inthe art to form the top capacitor electrode may be used.

A third exemplary implementation of the present invention is similar tothe first example except high dielectric constant materials are used forthe cell dielectric. In that regard and referring now to FIG. 3A, across-sectional view depicts a capacitor storage electrode 31 onsemiconductor substrate 30. As before, semiconductor substrate 30 ismaking reference to a material which supports the construction of thecapacitor electrode 31. Capacitor electrode 31 consists of a polysiliconportion 32 on top of which is formed of TiN so that capacitor electrode31 has a TiN surface 33. Although polysilicon is shown and is apreferred material used to construct the lower portion of capacitorelectrode 31, other materials may be used that are suitable foraccepting a layer of TiN as the surface layer. After the TiN is formed,a conductive layer 34, being any one of strontium, titanium or astrontium/titanium compound, is formed directly on the TiN layer 33.Layer 34 may be formed by sputtering or chemical vapor deposition to apreferred thickness of less than 50 Å. As in the first embodiment, thethickness of layer 34 is critical as it must be thick enough to protectthe underlying TiN layer 33 from becoming oxidized during any subsequentexposure to oxygen.

Referring now to FIG. 3B, conductive layer 34 is converted to oxide 35during an oxygen anneal. As discussed in the first example, theconversion of the conductive layer to an oxide may either be performedby an oxygen anneal while in a deposition chamber which will occurduring a subsequent oxide deposition step or by a separate rapid thermaloxidation step prior to the oxide deposition. Regardless of the methodused to oxidize the conductive layer (again, any one of strontium or astrontium/titanium compound), major advantages of the present inventionare gained. For example, using strontium/titanium, thestrontium/titanium, will initially make a strong chemical bond to theunderlying TiN during the formation. The oxidation of thestrontium/titanium layer maintains a strong chemical bond to the TiN andalso advantageously provides a strontium titanate (STO) nucleationsurface for a subsequent deposition of a high dielectric constantmaterial, while protecting a majority if not 100% of the TiN frombecoming oxidized.

Referring now to FIG. 3C, a high dielectric constant material to serveas cell dielectric 36, is deposited on strontium oxide nucleationsurface 35. In this particular implementation, barium strontium titanate(BST) or strontium titanate (STO) are ideal selections due to thepresence of a chemically similar nucleation surface, mainly strontiumtitanate. As mentioned above, the previous oxidation of thestrontium/titanium creates a nucleation surface that is chemically thesame as the cell dielectric that is deposited. Thus the deposited highdielectric material forms a strong chemical bond to the strontiumtitanate nucleation surface and thereby creates a high quality celldielectric film which has been shown to reduce charge leakage and thusresults in capacitors with excellent reliability and electricalperformance. Finally the capacitor is completed by the formation of atop electrode 37. It is preferred that electrode 37 be titanium nitride,however other materials used by those skilled in the art to form the topcapacitor electrode may be used.

In the above exemplary implementation, the same idea may be applied ifother high dielectric constant materials, such as, lead lanthanumtitanate (PLT), lead lanthanum zirconium titanate (PLZT), etc., areselected. An important aspect of the present invention is to provide anucleation surface for a subsequent dielectric deposition so that thenucleation surface and the subsequently deposited dielectric possesscommon chemical elements.

Ideally, for all of the exemplary implementations of the presentinvention it is preferred that a maximum TiN surface oxidation of 50 Åresults and it is further preferred none of the TiN surface becomesoxidized. It is also preferred that the overall cell dielectricthickness (which includes the oxidation or nitridation of the barrierlayer plus the deposited dielectric) be no greater than 100 Å, with theideal thickness being around 75 Å. However, for high dielectric constantmaterials, such as BST or STO, a desired overall thickness will be lessthan 500 Å. Taking the desired overall cell dielectric thickness intoaccount, a barrier layer being no greater than approximately 50 Å willprovide adequate protection for the underlying TiN as well as result inthe desired overall cell dielectric thickness. Though 50 Å has beenshown to be sufficient, process parameter variations will dictate theoptimum barrier layer thickness used and thus the present invention isnot limited to the above film thickness limitations. but rather providesa method to use a barrier film of optimal thickness.

The present invention has been described in several exemplaryimplementations wide reference to a bottom capacitor electrode having aTiN surface. Though it is preferred that the capacitor have a TiNsurface, it is not intended to limit the scope of the present invention.For example, the method taught herein is also applicable to silicon aswell as all refractory metal and refractory metal nitrides (i.e.,tungsten, tungsten nitride, tantalum and tantalum nitride). Furthermore,the method of the present invention is also applicable to a capacitorelectrode that has a noble metal surface and in particular platinum orruthenium. In the case of a noble metal, though the noble metal itselfis immune to oxidation, the presence of a barrier layer will preventoxygen atoms from diffusing through the noble metal and oxidizing anunderlying layer, such as silicon.

It is to be understood that although the present invention has beendescribed with reference to several preferred embodiments, variousmodifications, known to those skilled in the art,. may be made to thestructures and process steps presented herein without departing from theinvention as recited in the several claims appended hereto.

What is claimed is:
 1. A method for bonding a dielectric material to aconductive material in a semiconductor fabrication process comprising:forming a conductive layer directly on said conductive material;converting a portion of said conductive layer to a dielectric compoundhaving a dielectric nucleation surface; after said step of converting,depositing a dielectric film directly on said dielectric nucleationsurface, said dielectric film, said dielectric nucleation surface andsaid dielectric compound containing common chemical elements.
 2. Themethod of claim 1, wherein said conductive material consists of arefractory metal.
 3. The method of claim 1, wherein said conductivematerial consists of a refractory metal nitride.
 4. The method of claim1, wherein said conductive material consists of a noble metal.
 5. Amethod for preparing a conductive material in a semiconductor assemblyto receive a dielectric comprising: forming a conductive layer on saidconductive material; converting a portion of said conductive layer to adielectric compound having a dielectric nucleation surface, saiddielectric nucleation surface and said dielectric compound containingcommon chemical elements.
 6. The method of claim 5, wherein saidconductive material consists of a refractory metal.
 7. The method ofclaim 5, wherein said conductive material consists of a refractory metalnitride.
 8. The method of claim 5, wherein said conductive materialconsists of a noble metal.
 9. A method for preparing titanium nitride toreceive a dielectric material comprising: forming a conductive layerdirectly on said titanium nitride; converting a portion of saidconductive layer to a dielectric compound having a dielectric nucleationsurface, said dielectric nucleation surface and said dielectric compoundcontaining common chemical elements.
 10. A method for limiting oxygendiffusion to a conductive film in a semiconductor fabrication processcomprising: forming a conductive layer directly on said conductive film;converting a portion of said conductive layer to a dielectric materialhaving a dielectric nucleation surface; after said step of converting,depositing a dielectric film directly on said dielectric nucleationsurface of said dielectric compound, said dielectric film, saiddielectric nucleation surface and said dielectric compound containingcommon chemical elements.
 11. The method of claim 10, wherein saidconductive material consists of a noble metal.
 12. The method of claim10, wherein said conductive material consists of a refractory metal. 13.The method of claim 10, wherein said conductive material consists of arefractory metal nitride.
 14. A method for limiting oxidation oftitanium nitride in a semiconductor fabrication process comprising:forming a conductive layer directly on said titanium nitride; convertinga portion of said conductive layer to a dielectric material having adielectric nucleation surface; after said step of converting, depositinga dielectric film directly on said dielectric nucleation surface of saiddielectric compound, said dielectric film, said dielectric nucleationsurface and said dielectric compound containing common chemicalelements.
 15. The method of claim 14, wherein said converting a portionof said conductive layer to a dielectric occurs during said depositing adielectric film.
 16. The method of claim 14, wherein said converting aportion of said conductive layer to a dielectric further comprisesannealing said conductive layer in the presence of a dielectric formingagent prior to said depositing a dielectric film.
 17. A process forforming a capacitor for a semiconductor assembly comprising: forming abarrier layer directly on a conductive capacitor electrode, said barrierlayer being of sufficient thickness to substantially limit diffusion ofoxygen atoms therethrough while in the presence an oxidizing agent;converting a portion of said barrier layer to a dielectric compoundhaving a dielectric nucleation surface; after said step of converting,depositing a storage cell dielectric directly on said dielectricnucleation surface of said dielectric compound, said storage celldielectric, said dielectric nucleation surface and said dielectriccompound being of substantially the same chemical makeup.
 18. A processfor preparing a capacitor conductive electrode to receive a dielectricduring semiconductor processing comprising: forming a barrier layerdirectly on said capacitor conductive electrode; converting a portion ofsaid barrier layer to a dielectric compound having a dielectricnucleation surface, said dielectric nucleation surface and saiddielectric compound containing common chemical elements.
 19. The processof claim 18, wherein said capacitor conductive electrode consists oftitanium nitride.
 20. The process of claim 18, wherein said barrierlayer comprises a material containing any one of amorphous silicon,tantalum, titanium, or strontium.
 21. The process of claim 18, whereinsaid barrier layer is no greater than 50 Å in thickness.